Digital modulation in optical communication systems needs a low-distortion amplifier according to a modulation system thereof. For example, U.S. Pat. No. 7,076,226 describes a differential amplifier with an improved linearity to obtain a low-distortion signal. As illustrated in FIG. 10, a differential amplifier 100 includes transistors 112, 113, 122, and 123. A pair of the transistors 112, 113 constitutes a differential pair 111 having non-linearity. Another pair of the transistors 122, 123 constitutes a differential pair 121 having non-linearity. The differential pair 121 and the differential pair 111 are connected in parallel to each other. The non-linearity of an output current Id11 output from the differential pair 111 and the non-linearity of an output current Id14 output from the differential pair 121 are canceled out by each other. Also, the non-linearity of an output current Id12 output from the differential pair 111 the non-linearity of an output current Id13 output from the differential pair 121 are canceled out by each other.
For example, technologies relating to a differential amplifier are described in Japanese Patent Application Laid-Open No. 1-261905, Japanese Patent No. 2915440, U.S. Pat. No. 5,227,681, and US Patent No. 2011/0304394. In optical transmission systems or the like constituting a core network, a superior low-distortion differential amplifier has been required for driving an optical modulator or the like, especially in a phase shift modulation system such as quadrature phase shift keying (QPSK).